Untitled

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>Untitled Document</title>
<style type="text/css">
#caja {
        background-color: #999;
        height: 300px;
        width: 300px;
        position: fixed;
        left: 0px;
        bottom: 0px;
}
body {
        background-color: #930;
}
#footer {
        background-color: #00F;
        height: 100px;
        width: 100%;
        position: fixed;
        bottom: 0px;
        opacity:0.8;
        left: 0px;
}
#top {
        background-color: #0C6;
        height: 100px;
        width: 60%;
        top: 0px;
        right: 0px;
        position: fixed;
}
#cuerpo {
        background-color: #0CF;
        height: 350px;
        width: 600px;
        right: 0px;
        position: fixed; /*el negativo de la mitad de la altura del div*/
        z-index: 2000;
}
</style>
</head>

<body>
<div id="top"></div>
<div id="cuerpo"></div>
<div id="caja"></div>
<div id="footer"></div>
</body>
</html>